1. Field of the Invention
This invention relates to a zinc-bromine battery, and more particularly, to improvements of an electrolyte circulation type zinc-bromine battery having electrolyte storage tanks.
2. Description of the Prior Art
A zinc-bromine battery is known as a new type of battery. The following fundamental electrochemical reactions take place in a reaction tank provided with an anode and a cathode of a zinc-bromine battery.
The reaction at the cathode is: EQU 2BR.sup.- .revreaction.Br.sub.2 +2e.sup.-
The reaction at the anode is: EQU Zn.sup.2+ +2e.sup.- .revreaction.Zn (1)
The cell reaction is: ##STR1##
As is clear from the reaction formulae, zinc Zn is deposited during charge and bromine Br.sub.2 which dissolves into the electrolyte is produced at the cathode. On the other hand, during discharge the zinc Zn deposited onto the anode is oxidized to Zn.sup.2+ and dissolves into the electrolyte, while the bromine Br.sub.2 in the electrolyte is reduced to bromine ion 2Br.sup.- and dissolves into the electrolyte.
In such a zinc-bromine battery, the concentration of the bromine Br.sub.2 in the electrolyte which is produced during charge increases as the charging time passes, and the bromine Br.sub.2 gradually diffuses toward the anode. The bromine Br.sub.2 reacts with the zinc Zn at the anode and becomes zinc ion Zn.sup.2+ and bromine ion Br.sup.-, thereby causing self-discharge. The zinc-bromine battery is therefore provided with a separator membrane which allows zinc ion Zn.sup.2+ and bromine ion Br.sup.- to permeate therethrough but which impedes the permeation of bromine Br.sub.2 in order to separate the reaction tank into anode and cathode reaction tanks, thereby preventing bromine Br.sub.2 from diffusing from the cathode side to the anode side.
Furthermore, in order to prevent diffusion of the bromine Br.sub.2, a complexing agent is added to the electrolyte of the zinc-bromine battery, so that the bromine Br.sub.2 dissolved into the electrolyte on the cathode side is converted into a complex compound which is insoluble in the electrolyte, and is deposited and precipitated in the form of oil in the electrolyte.
FIG. 3 shows a conventional zinc-bromine battery produced on the basis of the above-described principle. In this battery, a cathode 12 and an anode 14 are provided within a reaction tank 10, one on each side thereof, whereby the electrochemical reaction indicated by the formulae (1) takes place between the cathode 12 and anode 14 through electrolytes 16.
In such a zinc-bromine battery, zinc-bromine (ZnBr.sub.2) aqueous solution is used as the electrolyte 16, and an electric conductance improver, a bromine complexing agent, a dendrite inhibiter and the like are added thereto as occasion demands.
During charge, the charging reaction shown in the formulae (1) takes place in the reaction tank 10, and bromine Br.sub.2 is generated on the cathode side 12 and dissolves into the electrolyte 16, while on the anode side 14 zinc Zn is deposited and a precipitation layer 18 is formed on the anode 14.
On the other hand, the reaction which is reverse to the charging reaction takes place during discharge. Bromine Br.sub.2 is reduced to bromine ion 2Br.sup.- on the cathode side 12 and dissolves into the electrolyte 16, while on the anode side 14 the zinc precipitation chamber 18 is oxidized to zinc ion Zn.sup.2+, and dissolves into the electrolyte 16.
The reaction tank 10 in which these electrochemical reactions take place is provided with a separator membrane 20 which divides the interior of the tank into a cathode reaction tank 10a and an anode reaction tank 10b so as to prevent any occurrence of self-discharge caused by the bromine Br.sub.2 which is produced during charge.
The separator membrane allows the electrolyte 16 to permeate therethrough but impedes the permeation of the bromine Br.sub.2 which is in solution in the electrolyte 16, so as to prevent any occurrence of self-discharge. An ion-exchange membrane or a micro-porous membrane is generally used as the separator membrane 20, but a micro-porous membrane is more preferable from the viewpoint of reducing the inner resistance of the battery.
In an electrolyte circulation type battery, a catholyte storage tank 22 and an anolyte storage tank 24 are provided in order to store the electrolyte.
Pipes 26 and 28 provided between the catholyte storage tank 22 and the cathode reaction tank 10a constitute an electrolyte circulation passage, and a pump 30 provided in the circulation passage delivers the catholyte 16a which has reacted in the cathode reaction tank 10a to the storage tank 22, and supplies new electrolyte 16a from the storage tank 22 to the reaction tank 10a.
In the case wherein a bromine complexing agent is added to the electrolyte 16a, the bromine Br.sub.2 generated during the charge is complexed, and is deposited as a complex compound which is insoluble in the electrolyte 16. This complex compound is subsequently precipitated and stored at the bottom of the storage tank 22 as a complex compound storing chamber 32, as is shown in the battery of FIG. 3.
The complex compound storing chamber 32 is connected to the pipe 28 by a complex compound supply pipe 36 having a valve 34. This valve 34 delivers the complex compound which has precipitated in the complex compound storing chamber 32 to the reaction tank 10a through the pipe 28 for the purpose of discharge.
Similarly, pipes 38 and 40 provided between the anolytic storage tank 24 and the anode reaction tank 10b constitute an electrolyte circulation passage, and a pump 42 provided in the circulation passage delivers the anolyte 16b which has reacted in the anode reaction tank 10b to the storage tank 24, and supplies new electrolyte 16b from the storage tank 24 to the reaction tank 10b.
In this way, this zinc-bromine battery can adequately store the electrolyte 16 in the storage tanks 22 and 24, cause the charging reaction shown in the formulae (1) in the stored electrolyte 16 during charge, store the bromine complex compound in the complex compound storing chamber 32, and form the zinc precipitation layer 18 on the anode 14, thereby storing electric power. During discharge, on the other hand, the zinc-bromine battery can deliver the bromine complex compound stored in the complex compound storing chamber 32 to the cathode reaction tank 10a, and cause the discharge reaction shown in the formulae (1) between the complex compound and the zinc precipitation layer 18 formed on the anode 14, thereby emitting the charged electric power.
Although a conventional zinc-bromine battery is capable of efficient charging and discharging in this way, it has the following problems which remain unsolved.
This kind of conventional zinc-bromine battery is completely divided into the cathode side and the anode side, and the catholyte 16a and the anolyte 16b are mixed with each other solely by permeation through the separator membrane 20. Since the separator membrane 20 has a predetermined resistance when the electrolyte 16 permeates it, it is impossible to store electric power by efficiently utilizing the zinc ions Zn.sup.2+ which are contained in the catholyte 16a during charge. It is also impossible to effectively utilize the complexing agent contained in the anolyte when a bromine complexing agent is contained in the electrolyte 16.
FIG. 4 shows the change in zinc ion concentration of the catholyte 16a and the anolyte 16b during charge.
As is obvious from the above-described reaction formulae, during charge the zinc ions Zn.sup.2+ are attracted to the anode 14 and zinc Zn is deposited.
At this time, since the cathode side is separated from the anode side by the separator membrane 20, it is much more difficult for the zinc ions Zn.sup.2+ in the catholyte 16a to move toward the anode 14 than the zinc ions in the anolyte 16b. Therefore, since the zinc ions in the anolyte 16b are consumed sooner than the zinc ions Zn.sup.2+ in the catholytic 16a and charging is completed at that point, it is impossible to perform charging by adequately utilizing the zinc ions contained in the catholyte 16a.
In particular, when a salt halogenide such as KCl is added to the electrolyte 16 as a supported salt in order to improve the electric conductance of the electrolyte 16, or when an electrolyte of high concentration (more than 3 Mol/L ZnBr.sub.2) is used the difference in concentration of the electrolytes 16a and 16b becomes greater. Consequently, at the last stage of charge, the zinc ions contained in the anolyte 16b are reduced to an extreme extent in comparison with those of the catholyte 16a, and the utilizing ratio of the zinc ions contained in the electrolyte 16 is further lowered.
FIG. 5 shows the zinc ion concentration when KCl is added to the electrolyte 16. As is clear from the graph, the difference in concentration of the zinc ions contained in the electrolytes 16a and 16b is greater in this case than the difference shown in FIG. 4, and it will be understood that the utilization ratio of the zinc ions is thereby further lowered.
This is because the KCl added to the electrolyte 16 in this way reacts with the zinc ion Zn.sup.2+ and becomes (ZnCl.sub.4).sup.2-, so tha some of the zinc ions which should intrinsically be plus becomes minus ions and are attracted toward the cathode side, thereby substantially lowering the concentration of the zinc ions contained in the electrolyte 16b.